VLAN tagging is a method in which a VLAN tag is added to an Ethernet header to help distinguish traffic from multiple networks carried over the same interface. In the architectures described so far in this book, an instance connected to multiple networks has a corresponding interface for each network. This works at small scale, but PCI limitations may cap the number of interfaces that can be attached to an instance. In addition, hot-plugging interfaces to running VMs when attaching new networks may have unexpected results.
The following diagram visualizes the concept of one vNIC per network:
In Figure 14.1, a single vNIC is associated with a Neutron port. Neutron typically performs VLAN tagging at the virtual switch based on the segmentation_id provider attribute of the respective VLAN network. In this case, instances are not expected to perform any...